1. Field of the Invention
This invention relates to a blow-molded bottle-shaped container of biaxially oriented polyethylene terephthalate resin and, more particularly, to a bottle-shaped container in which large durable strength is created against an increase in the pressure in the bottle-shaped container but which is easily and uniformly deformed under reduced pressure in the container.
2. Related Art
It is known that a blow-molded bottle-shaped container of biaxially oriented polyethylene terephthalate resin (hereinafter referred merely to as "PET") is improved in the heat resistance of the container body itself by heat setting the resin after biaxial-orientation blow-molding to provide a heat resistant bottle-shaped container for filling content liquid necessary to be filled at high temperature, such as juice drink.
However, the bottle-shaped container of PET of this type does not have high rigidity like a glass or metal bottle-shaped container, but is flexible. Thus, the body of the bottle-shaped container is improperly deformed under reduced pressure generated in the container due to a volumetric contraction of content liquid or a decrease in the vapor pressure of a head space when filling the content liquid at high temperature to cause the container to be remarkably defected in its external appearance.
The bottle-shaped container of the PET of this type is prevented from being deformed in the configuration of the body by recessing and aligning flat longitudinal reduced pressure absorbing panels on its body to absorb the reduced pressure in the container by means of the panels.
Pressure and stress are acted on the panels of the heat resistant bottle-shaped container of the PET as below. Hydraulic pressure produced due to the difference in height of the surface of the content liquid filled in the container from the content liquid in a tank disposed at its upper position at the time of pressing to seal the neck of the container and filling the liquid content in the container by a filling machine in case of filling the content liquid at high temperature is acted on the panels of the container. The hydraulic pressure is opened with the atmospheric pressure immediately after filling the content liquid in the container. A rise in the internal pressure in the container due to vapor pressure in the head space of the container at the time of capping the neck of the container (e.g., the internal pressure in the container is raised to approximately 1.7149 kg/cm.sup.2 when the content liquid of 90.degree. C. is, for example, filled in the container). The vapor pressure in the container is reduced gradually from the state at capping time to the atmospheric pressure at sterilization time, and the pressure in the container is decreased in the deforming stress in response to the pressure change caused by the content from being reduced in volume at cooling time and to the reduction in the vapor pressure in the head space of the container. The deforming stresses are generated at the panels in response to the pressure change.
As described above, the panels are affected by the heat from the content liquid in the container and also subjected to the pressure change at pressurizing time (at the time of filling the content or capping the neck of the container), to the ambient pressure (immediately after filling the content liquid in the container) or to the pressure reduction (at the time of cooling the container). Therefore, the panels are heated to high temperature and pressurized to high pressure at the time of filling the content in the container, capping the neck of the container due to the vapor pressure and the heat of the content liquid immediately thereafter, and thus extrusion-deformed in a raised shape at the outside of the container as compared with that at the time of vacant container.
According to a number of experiments, generated vapor pressure is relatively low when the temperature of the content liquid to be filled is 80.degree. C. or lower, so that the temperature rising degree of the container is less. Thus, the allowable stress to the container itself is still large, a trend that the panels are deformed in a raised shape is relatively small, and the influence of the raised deformation of the panel is not almost presented after cooling the container. However, when the temperature of the content liquid is 85.degree. C. or higher and particularly 90.degree. C. or higher, generated vapor pressure in the container is raised, and the raised deformation of the panel after capping the neck of the container is much increased.
Since the raised deformation of the panel of the container is affected by the influences of the temperature of the content liquid and the vapor pressure of the container, a permanent strain remains in the material of the container due to a decrease in the strength of the material and the remaining strain.
The panels provided on the bottle-shaped container of this type are heretofore composed, in order to obtain uniform deformation, of (1) flat surfaces as large as possible on the entire area of the panels, (2) external projections of the entire panel in advance, (3) external protrusion of partial panel in advance, (4) inclined surfaces of the panels to reduce the raised deformation, (5) recess grooves surrounded on the panels to scarcely cause the panels to be deformed in a raised shape, and (6) lateral and longitudinal rib strips formed on the panels. However, when the temperature of the content liquid filled in the container is actually raised to 85.degree. C. or higher, raised deformations indispensably generated on the panels are increased due to the influences of the heat and vapor pressure of the liquid content in the container, and permanent deformations remain at the panel as remaining strains at the time of cooling the container. The panels which have once been subjected to the raised permanent deformations cannot function as ordinary panels and lose their reduced pressure absorbing action. Thus, the entire body of the container is improperly deformed to triangular or elliptical shape, or the panels cannot absorb the normal pressure reduction, thereby causing the external appearance of the container to be deteriorated.
As described above, it is also known that panels which cause less raised deformation against an increased pressure at the time of capping the neck of the container and also cause easy deformation due to recessed deformation under reduced pressure in the container at the time of cooling the container are formed in flat structure in the whole inside of the stepped portion of the panels surrounded by bent stepped portions on the periphery. However, mere flat structure of the entire panel causes the stepped portions to be subjected to permanent deformations as will be described so that the panels cannot absorb deformations due to normal reduced pressure. Even if the panels may absorb the reduced pressure deformation, the available state of the stress acting on the panels due to the reduced pressure cannot be specified to be uniformized. Thus, predetermined stable deformations cannot be proceeded at the panels. In this manner, the degrees of absorbing the deformation due to reduced pressure in the panels become different, so that the external appearance of the bottle-shaped container is abnormally deteriorated.
The most simple means which do not retain permanent deformations in the raised strains of the panels is to raise the heat setting effect of the container. The heat setting includes biaxial-orientation blow-molding a preformed piece by injection molding, then cooling the piece, then heating again the piece to remove its remaining stress, and thereafter further blowing the piece to complete a product. However, in order to raise the heat setting effect of the bottle-shaped container, it is necessary to raise the heat setting temperature and to increase the setting time. Thus, the heat setting remarkably reduces the productivity. Therefore, a method of raising the heat setting is not practical. Even if the container is sufficiently heat set in this manner, the deformation for the reduced pressure absorbing effects of the panels cannot be always uniformly generated, but a decrease in the external appearance of the container due to irregular deformation still remains unsolved.
Since blow-molded bottle-shaped container of biaxially oriented synthetic resin is removed from a metal mold in the state the container is yet soft after blow-molding, the container is feasibly deformed due to small remaining distortion. This distortion of the container is understood to be largely affected by the structure of the panels. The bottle-shaped container having conventional panels as described above has remarkable drawbacks to be readily deformed in its structure after blow-molding.
The causes of the permanent deformation of the panel in the bottle-shaped container have been observed in detail. It is discovered that one of the causes resides in the fact that the bending angles of two bent portions of the stepped portions bent at the periphery of the panels are varied in opposite directions each other to be different from the angle at the time of molding.
The variations in the bending angles of the two bent parts of the stepped portions was understood from the fact that permanent deformations occurred due to the excess of allowable range of the deformations varied in opposite directions at the two bent parts by the temperature and the vapor pressure of the liquid content to be filled. When the stepped portions are thus deformed, the entire panels remain deformed in raised shape, resulting in impossibility of smoothly recessed distortion for absorbing reduced pressure in the container.
In a cylindrical bottle-shaped container, the body is located at equal distances from the center line at any portion. Thus, the container is easily uniformly oriented. However, in a polygonal bottle-shaped container, the body is not located at equal distances from the center line; according to the positions, the container is subjected to irregular orientations Therefore, the amounts of orientations are different at the positions on the container. Thus, internal remaining stresses generated by blow-molding are at different positions on the body. The differences in the blow-molding cause the panels to be subjected to permanent deformations at the time of heat setting or completing the container. This is also remarkable particularly at the bottom of the container at the portions which are most feasibly affected by the orientations.